Indirect Effects of Feral Horses on Estuarine Communities

Abstract: Livestock have grazed on salt marshes for centuries and have dramatic effects on marsh vegetation. Most studies examining the effects of livestock on salt marshes have focused on the effects on plants rather than on salt marsh fauna or ecological processes. However, grazers such as feral horses may have strong indirect effects on communities by altering the habitat, making it more or less suitable for species that potentially occur there. We evaluated the indirect effects of grazing by feral horses on estuarine animals that use salt marshes and adjacent subtidal communities. Surveys revealed that horse‐grazed marshes had less vegetation, a higher diversity of foraging birds, higher densities of crabs, and a lower density and species richness of fishes than marshes not grazed by horses. In addition, fish density was reduced in subtidal habitats adjacent to grazed marshes. Experiments manipulating marsh vegetation indicated that the potential for predation on fishes in ungrazed marshes was higher than in grazed marshes. Results of additional experiments in which fishes were enclosed with or without artificial Spartina suggested that the removal of shelter provided by marsh vegetation results in behavioral shifts by fishes that make them more susceptible to predation. Although large herbivores are naturally absent from extant salt marsh ecosystems, such large herbivores were common members of Pleistocene communities. Using modern horses as surrogates for extinct ungulates, we hypothesize that large herbivores could have had strong indirect effects on Pleistocene estuarine habitats. We argue that both the modern introduction of ungulates to salt marshes and the prehistoric elimination of large herbivores affected estuarine biodiversity.